1. Field of the Invention
This invention relates to a power conversion system, and more particularly to a voltage-type self-commutated conversion system such as a self-commutated reactive power compensator device which performs voltage control, reactive power control and so forth of a power system and a self-commutated power conversion system which performs power interchange by means of DC transmission and frequency conversion.
2. Description of the Related Art
FIG. 4, a diagram for the purpose of explaining the prior aft, shows a voltage-type self-commutated power converter 1 (referred to simply as a converter below) which connects to an AC system 4 via a 3-phase system connection transformer 3, together with its control devices. Element 2 is a DC capacitor, 8, 9 and 10 are current transformers (CTs), 11 is a converter output voltage reference computation circuit, 12 is a synchronization detection circuit, 13 is an active current setting device, 14 is a reactive current setting device, 15 is a PWM gate control circuit and 20 is a protection circuit for the converter 1.
In FIG. 4, the converter 1 is composed of self-turn-off devices GU, GV, GW, GX, GY and GZ, for example GTOs (gate turn-off thyristors), power transistors, static induction thyristors and other power electronic devices with a self-turn-off function, together with diodes, DU, DV, DW, DX, DY and DZ connected respectively in anti-parallel with each self-turn-off device.
Hereinafter, an explanation will be given in the case where the converter 1 is operated as an inverter for converting DC input power into AC output power. The 3-phase output voltage of the converter 1 in a system with this kind of construction can be controlled by varying the conducting period of the self-turn-off devices, GU, GV, GW, GX, GY and GZ. The current received from or delivered to the AC system 4 via the impedance of the system connection transformer 3 is controlled by adjusting the phase angle and amplitude of the 3-phase output voltage of the converter 1 in accordance with the phase angle and amplitude of the system voltage VR, VS and VT of the AC system 4. By this means, the voltage-type self-commutated conversion system composed of the converter 1, the DC capacitor 2 and the system connection transformer 3 can either exchange active power with the AC system 4 by converting DC power to active power or compensate the reactive power of the AC system 4.
The converter output voltage reference computation circuit 11, the synchronization detection circuit 12, the active current setting device 13, the reactive current setting device 14 and the PWM gate control circuit 15 constitute a control device for the voltage-type self-commutated conversion system which controls the active power and reactive power.
The synchronization detection circuit 12 detects the system voltage phase angle .theta. of the system voltages VR, VS and VT of the 3-phase AC system 4. The converter output voltage reference computation circuit 11 computes converter output voltage references VRc, VSc and VTc which determine the output voltage of the 3 phases of the converter 1 in order to regulate the converter output AC currents iR, iS and iT detected by the current transformers 8, 9 and 10 in accordance with an active current reference iqc from the active current setting device 13 and a reactive current reference idc from the reactive current setting device 14.
The converter output voltage reference computation circuit 11 determines the phase angles of the converter output voltage references VRc, VSc and VTc to the system voltages VR, VS and VT based on the system voltage phase angle .theta. detected by the synchronization detection circuit 12.
The PWM gate control circuit 15 outputs gate signals U1, V1, W1, X1, Y1 and Z1 which determine the conducting periods of the self-turn-off devices GU, GV, GW, GX, GY and GZ of the converter 1 by comparing the converter output voltage references VRc, VSc and VTc with a triangular wave carrier signal generated based on the system voltage phase angle .theta..
A protection signal P1 is generated from a protective relay element (not shown) in order to protect the converter 1 from overcurrent, overvoltage and so forth.
The construction of the protection circuit 20 will be described with reference to FIG. 5. The gate signals U1, V1, W1, X1, Y1 and Z1 from the PWM gate control circuit 15 are applied to first input terminals of AND circuits 20U, 20V, 20W, 20X, 20Y and 20Z, respectively. The protection signal P1 is applied to second input terminals of the AND circuits 20U, 20V, 20W, 20X, 20Y and 20Z through an inverter circuit 20I. The AND circuits 20U, 20V, 20W, 20X, 20Y and 20Z generate gate signals Ug, Vg, Wg, Xg, Yg and Zg, which are applied to gates of the self-turn-off devices GU, GV, GW, GX, GY and GZ of the converter 1, respectively.
When the protection signal p1 is not present, the protection circuit 20 generates the gate signals U1, V1, W1, X1, Y1 and Z1 as the gate signals Ug, Vg, Wg, Xg, Yg and Zg to the converter 1 to control the conducting periods of the self-turn-off devices GU, GV, GW, GX, GY and GZ, respectively. When the protection signal P1 is generated, the protection circuit 20 stops the gate signals U1, V1, W1, X1, Y1 and Z1 of the PWM gate control circuit 15 and generates gate signals Ug, Vg, Wg, Xg, Yg and Zg to turn off all the self-turn-off devices GU, GV, GW, GX, GY and GZ in order to protect the converter 1 from overcurrent, overvoltage and so forth in accordance with the protection signal P1 from a protective relay element not shown in the diagram.
The conventional voltage-type self-commutated conversion system shown in FIG. 4 suffers from the types of problems described below. Because the output voltage of the converter 1 and the AC voltage diverge, if the AC voltage is distorted owing to the introduction of the power capacitor or the transformer and so forth, which are connected to the AC system 4 and not shown in the diagram, the output AC current of the converter 1 can increase and become an overcurrent. When this happens, the overcurrent relay not shown in the diagram operates and the self-turn-off device GU, GV, GW, GX, GY and GZ are all turned off by the protection circuit 20.
FIG. 6 shows the state of the converter 1 immediately before the protection action. The dashed lines in FIG. 6 show the flow of current immediately before the protective action; for example, R-phase current is flowing in towards the + (positive) side DC bus line P of the converter 1 from the AC system 4 through the diode DU. The self-turn-off device GV has been turned on by PWM control, and S-phase current returns to the AC system 4 through the self-turn-off device GV. T-phase current returns to the AC system 4 from the DC capacitor 2 through the - (negative) side DC bus line N and the diode DZ.
If the R-phase current becomes an overcurrent in the situation of FIG. 6, the self-turn-off devices GU, GV, GW, GX, GY and GZ are all turned off by the action of the protection circuit 20 of FIG. 4. When this happens, S-phase current becomes unable to flow through the self-turn-off device GV and returns to the AC system 4 via the + (positive) side DC bus line P, the DC capacitor 2 and the diode DY. This flow of current is shown by a solid line.
The flow of current shown by the solid line in FIG. 6 is in the direction which charges up the DC capacitor 2. Since this charging current is a large one, sufficient for the overcurrent relay not shown in the diagram to operate, the DC voltage rises to some extent. Because of this, a DC overvoltage is generated and there is a risk of damage to the converter 1 or devices connected to the DC bus lines P and N.
With a conventional control and protection system, there is therefore, contrary to what is intended, a risk of generating a DC overvoltage as a result of the protective action of the converter. The withstand capability to overvoltage of a semiconductor device is inherently less than that to overcurrent, and it becomes damaged if the overvoltage exceeds its withstand capability even instantaneously. There is therefore a considerable risk of damaging the converter by the protective action instead of protecting it. There is consequently a danger of being unable to restart a voltage-type self-commutated conversion system after a voltage waveform distortion has abated, and the system's essential purpose, voltage control and reactive power control of power system or a power interchange, becomes impossible.